The present invention relates to a method of preparing an etch resistant pattern on a substrate. In particular, this invention relates to a method of physically transferring a preformed pattern to a substrate. The method is useful in the formation of micro-patterns and in the fabrication of integrated circuitry.
In the fabrication of integrated circuitry and micron-scale devices, patterns are generally defined by photolithography. In order to define a pattern on a semiconductor wafer, a photoresist is used to produce a resist pattern on the substrate prior to image forming modifications of the substrate material, such as vapor deposition, ion implantation or etching.
Photoresist material is commonly applied to a wafer substrate by spin coating and is then dried to produce a thin film with a thickness of about 1 .mu.m. This dried film is subjected to photo imaging, for example, pattern reproduction from a mask in a photo printer. Various types of photo printers are used in the industry. One type of photo printer is a contact printer, where the photoresist layer is in direct contact with the pattern carrying mask. This method, which minimizes diffraction of light, provides the best image reproduction. However, the possibility of damage to the mask, and the difficulty in its alignment, limit the applications of this method.
Another type of photo printer is a projection printer. In this type of printer delicate optics are used to project the pattern from the mask onto the photoresist layer on the substrate. During the fabrication of fine line patterns, by the method, light diffraction problems are minimized through the use of reduction projection optics and an enlarged mask pattern. The magnitude of the reduction is usually by a factor of five or ten. However, such a method also shrinks the imaging field. Therefore, a step and repeat imaging mode with lower throughput is required. This type of projection printer also provides accurate overlay alignment and is capable of sub-micron imaging when improvements on the higher numerical aperture value of the optical system are made and shorter radiation wavelength is used.
After patterning, the latent image in the photoresist layer is developed in an appropriate solvent. In a positive working photoresist layer, the exposed area is washed off by the developer, while in a negative working photoresist layer, the unexposed area is washed off in the developing stage. In general, positive photoresist layers are more widely used to achieve higher resolution capability.
A diazoquinone/novolac system has been the most widely used positive working photoresist material. Diazoquinone, a photo-active compound, is converted into indene carboxylic acid upon exposure to photo-radiation. This conversion allows the development of the photoresist film in an aqueous alkali solution.
In negative working photoresists polymerization occurs in the irradiated region which makes the material insoluble. The pattern is developed by removal of the non-polymerized portion of the photoresist layer. Typical negative type photoresist materials include azide-sensitized rubber, such as cyclized polyisoprene, and polyvinylcinnamate with benzothiazole as photo-sensitizer.
During the production of integrated circuits, the patterning of the photoresist and the subsequent processes are repeated several times on a semiconductor wafer substrate. The non-planar circuit pattern topography on the substrate results in a non-planar photoresist coating. This causes focusing problem during photo-imaging of the coating. The use of a planarization coating prior to the application of a photoresist layer or multiple coatings of photoresist minimizes this problem, at the expense of a slower production rate, due to the time consuming extra coating and drying steps. However, the variation of the thickness of photoresist layer on the substrate makes it difficult to control the pattern profile. Furthermore, the different photo-reflectivity of various materials and the scattering of the reflected radiation off of the substrate topography, creates problems with the side wall profile and results in dimension control of the photoresist line becoming difficult. As a consequence, side wall indentation, line width variation over step topography of the substrate, and standing wave effects are commonly encountered during conventional image formation. The inclusion of a radiation absorbing dye or the use of an antireflective undercoat has been shown to minimize the influence of the substrate but with the problems of reduced photo-sensitivity and a narrowed processing window.
In view of the above problems, the use of dry developing in conjunction with the concept of "top imaging" emerged. In dry developing, a selective plasma etching reaction replaces the traditional wet developing process. In the top imaging process, a pattern is defined in the top photoresist layer of a multilayer system or in the top surface region of a single layer photoresist.
In the multilayer process, usually a two or three layer design, a thin layer of photoresist usually is placed on top of planarization sublayers. The pattern is initially defined in the top layer photoresist by a conventional method, that is, by wet developing. In a two layer system, the top photoresist layer is oxygen plasma etch resistant. Thus, in subsequent plasma etching, particularly oxygen plasma etching or oxygen reactive ion etching, the exposed sublayer is etched off and the top pattern is translated down to the inorganic substrate. However, there are problems associated with intermixing the etch resistant photoresist layer and the sublayer . These problems lead to difficulties in pattern definition control.
In the three layer system, a barrier layer is used which is located between the top photoresist layer and the bottom planarization layer. The barrier layer can be made of an inorganic compound material, preferably silicon oxide, siloxane, silane or other metals. Once the top pattern is defined, the exposed portion of the second layer is removed, usually by fluorocarbon plasma etching or a wet etching method which does not attack the top photoresist layer. Dry developing is used to translate the pattern down to the substrate. Although the multilayer systems exhibit high resolution, they suffer from the drawback of time consuming multiple coatings.
As a result, simpler single layer photoresists which are capable of dry developing were created. An oxygen plasma etchable photoresist is rendered selectively etch resistant by the imagewise inclusion of an etch resistant material. This material is usually organo-metallic compound which is introduced after photo-patterning of the photoresist layer. The selective inclusion of the organo-metallic compound is aided by the photo-reaction which has occurred in the imaged area. A typical example is a diazoquinone/novolac photoresist system. In this system, the exposed area allows a silicon compound to penetrate into the photoresist surface. This reacts with the novolac resin to form an etch resistant thin layer which can withstand oxygen plasma or oxygen reactive ion etching treatment. The use of this method allows the photoresist layer to be photo-patterned only in the surface region and avoids the problem of reflection of radiation from the substrate. However, a planar photoresist surface is still required to avoid the previously noted focusing problem. As a result, multiple coatings of photoresist may be required to cover the substrate topography.
In general, these existing advanced microlithography methods still suffer from the problems for using 3 complicated multiple photoresist patterning on what is usually a non-planar substrate. In addition, time consuming problems of alignment and focusing in the step and repeat mode of a projection printer also exist.